The invention relates to a rotary and percussive concrete drill with hard material elements that can be driven by a hand tool machine.
Conventionally, a small concrete drill up to a bore diameter of 40 mm driveable by a hand tool machine, to which the invention is limited, is configured either in one piece as a hammer drill with a fixed drill head comprising hard material elements or in two pieces with a drill head that is axially displaceable relative to the drill shaft and equipped with hard material elements. Such percussively and rotary stressed hard material elements usually have a blunt blade with a cutting radius greater than 1/20 of the bore diameter and a negative rake angle of −25° to −45°, wherein high impact energies are required for a predominantly grinding degradation process and wherein the rebar imbedded in the concrete are only inefficiently bored. The high airborne and impact noise is particularly prevalent in the construction and allied trades.
According to U.S. Pat. No. 4,852,672, a small concrete drill comprises a purely rotary cutting part and an impact part passing separately through the drill shaft, which creates a pilot bore hole that advances axially for relieving stress on the bore hole surface. According to DE 19748987, a large stone drill with a purely rotary cutting part equipped with hard material elements is configured with an impactable ram arranged within the hollow central tube of the drill shaft, wherein the ram creates a pilot bore hole that advances axially for relieving stress on the bore hole surface. The same degradation process constantly acts on a fine structure zone in the bore hole surface by virtue of this spatial separation of function one.